The current state of the art in noise suppression uses either passive systems or active systems. The passive systems either reflect the sound away, as with a wallboard in a house, or absorb the sound using blankets or porous materials. The active noise suppression systems use a microphone or accelerometer to detect sound and a loudspeaker to re-radiate sound that is out of phase with the original signal.
Sound propagation losses in typical passive materials follow a principle commonly referred to as the “mass law.” In its most simple form, the mass law applies to air-borne sound impinging normally on a wall and states that the amount of sound transmitted into the air on the opposite side is a simple function of the mass density of the wall. This law holds because heavy materials reflect sound back towards the source more efficiently. Using this law, if sound is to be excluded from a room then the walls should be made out of very heavy material. However, this is not practical in many weight sensitive applications such as aircraft, spacecraft or automobiles.
Active noise suppression systems inject sound into the system that is 180° out of phase with the offending noise. Such systems work best when either the sound comes from a point source or the sound is cancelled at a specific point. In both cases, only one microphone and speaker, with the accompanying phase change electronics, are required. If sound comes from an extended area (such as through a wall) or it is desired to cancel the sound everywhere in a large volume, then active systems require too many electrical components to be practical.
It would be useful to be able to provide a passive material that selectively changes the phase of transmitted sound over a large area. The sound passing through this material could then be made to destructively interfere with the normally transmitted sound to cancel the sound everywhere in a large volume. Because such a material does not reduce sound by reflecting it, the mass law does not apply and a light material can be employed.